Other planets have been spotted in the habitable zones of their stars, but most of those worlds are Jupiter- or Neptune-size bodies that are unlikely to harbor life.

"The number of confirmed sub-Neptunian worlds in their habitable zones are few and far between, because they are the hardest ones to find," said Natalie Batalha, Kepler's deputy science team leader at San Jose State University in California.

In fact, only two known planets fit this description so far—Gliese 581d and HD 85512—and both worlds orbit at the very edges of their stars' habitable zones, making them more akin to Venus and Mars than to Earth.

"What makes this particular discovery so exciting is that this planet is right smack in the middle of the habitable zone," Batalha said.

"It's also orbiting a star that's almost a twin of our sun, whereas the other two detections are orbiting significantly cooler stars."

The Kepler mission finds new worlds by simultaneously monitoring 150,000 stars for dips in brightness, which are indicative of planets passing in front of—or transiting—their stars.

Kepler-22b was among the 54 roughly Earth-size planet candidates announced by the Kepler team in February. But the spacecraft needs to watch at least three transits to confirm that a signal is a planet.

"Fortune smiled upon us with the detection of this planet," William Borucki, Kepler's principal investigator at the NASA Ames Research Center in Moffett Field, California, said in a statement.

"The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season."

The new planet is about 2.4 times the radius of Earth, but scientists don't yet know its composition, because they are still missing a crucial piece of information: Kepler-22b's mass.

The Kepler team is hopeful, however, that the mass of Kepler-22b could be calculated with the help of a new ground-based instrument in the Spanish Canary Islands that will begin observations next spring.

Called HARPS North, the new telescope is capable of measuring with high precision a planet's doppler velocity—changes in the frequency of light from an object in space as it moves toward or away from Earth.

With this information, scientists can calculate the mass, and therefore the density, of Kepler-22b and determine whether it's a rocky planet or a water world.

"We are really hopeful that HARPS North might be able to be a really big help in this quest for the mass of this planet," San Jose State's Batalha said.

"We're just getting closer and closer to what is truly Earthlike, and that progress is exciting to watch."